The amplification of optical signals in fiber-optic telecommunication systems is achieved primarily through the use of discrete optical amplifiers, mainly erbium-doped fiber amplifiers (EDFAs), and through distributed Raman amplification (DRA) in which the transmission fiber itself is used as the amplifying medium.
Discrete amplifiers placed as boosters or pre-amplifiers at either end of an optical cable link are sufficient for moderate span lengths and capacities. However, as capacity and/or span length requirements increase in unrepeatered systems, distributed Raman amplification is typically implemented first and then, when even this is not sufficient, architectures with discrete EDFA amplifiers placed out in the cable and pumped remotely from the terminals are used. According to the prior art, these remote optically pumped amplifiers (ROPAs) are pumped by launching high power at 1480 nm from either the receiving terminal of the link in the case of a remote preamplifier or the transmitting terminal for a remote post amplifier. In the latter case, the 1480-nm power is delivered through one or more dedicated pump fiber(s) to avoid any interactions between the launched signal channels and the pump. A remote preamplifier can be pumped via the transmission fiber itself or a dedicated pump fiber or both.
The increase in link budget achievable through the addition of remotely-pumped amplifiers is determined by the maximum distance over which the required ROPA pump power can be delivered. Increasing the amount of pump power launched increases this distance, up to a point. As the launched 1480-nm pump power is increased, the resulting high Raman gain in the 1590-nm region begins to deplete the 1480-nm power delivered to the ROPA via the build-up of Raman noise and eventually, oscillations around 1590 nm. As pointed out by Boubal et al., SubOptic'2001, Kyoto, paper P3.6 (May 2001), the maximum 1480-nm launch power in standard pure silica core fiber (PSCF) for example, is ˜1.3 W. In other words, for the effective transmission of the 1480-nm pump energy to the ROPA, the Raman properties of optical fiber place an upper limit on the maximum 1480-nm launch power. This limit can be increased to ˜1.9 W by utilizing a segment of Large Effective Area PSCF (E-PSCF) leading away from the 1480-nm launch terminal. See for instance, E. Brandon et al., SubOptic'2001, Kyoto, paper T3.4.1 (May, 2001).
A further increase to ˜4 W has been demonstrated by incorporating fused WDM couplers in dedicated pump fibers to act as filters with low loss at 1480 nm but high loss in the region of high Raman gain. See for instance, Boubal et al., SubOptic'2001, Kyoto, paper P3.6 (May 2001). In a recent development, this same group combined the use of a hybrid PSCF/E-PSCF dedicated pump fiber incorporating fused WDM coupler filters with a first-order Raman pumping scheme to further increase the distance over which the required pump power could be delivered to a remote preamplifier. See L. Labrunie et al., Electronics Letters, Vol. 39, No. 19 (September, 2003). In this scheme, they launched high power at 1387 nm along with substantially lower power at 1480 nm from a laser diode. The high power at 1387 nm provided Raman gain for the 1480-nm power as it propagated along the dedicated pump fiber and resulted in a net increase in the 1480-nm power reaching the ROPA compared to a conventional pumping scheme involving the direct launch of high power at 1480 nm alone.
Despite the improvements in delivered pump power provided by these latter developments, they require dedicated pump fibers even for a remotely-pumped preamplifier, a fact which, in and of itself, has significant cost implications. Furthermore, the dedicated pump fiber is a hybrid PSCF/E-PSCF fiber incorporating WDM coupler filters, which adds to the complexity and cost of the pump fiber. In today's cost-sensitive environment, there is an ever-present need for performance improvements which have the least possible negative impact on the cost of fiber-optic communications systems. Thus, a ROPA pumping scheme, such as that disclosed in this application, which increases the pump power deliverable to remote amplifiers and, especially in the case of remote preamplifiers, does so with minimal impact on cost, is highly desirable.